Medical imaging is one of the most useful diagnostic tools available in modern medicine. Medical imaging allows medical personnel to non-intrusively look into a living body in order to detect and assess many types of injuries, diseases, conditions, etc. Medical imaging allows doctors and technicians to more easily and correctly make a diagnosis, decide on a treatment, prescribe medication, perform surgery or other treatments, etc.
There are medical imaging processes of many types and for many different purposes, situations, or uses. They commonly share the ability to create an image of a bodily region of a patient, and can do so non-invasively. Examples of some common medical imaging types are nuclear medical (NM) imaging such as positron emission tomography (PET) and single photon emission computed tomography (SPECT). Using these or other imaging types and associated machines, an image or series of images may be captured. Other devices may then be used to process the image in some fashion. Finally, a doctor or technician may read the image in order to provide a diagnosis.
A PET camera works by detecting pairs of gamma ray photons in time coincidence. The two photons arise from the annihilation of a positron and electron in the patient's body. The positrons are emitted from a radioactive isotope that has been used to label a biologically important molecule like glucose (a radiopharmaceutical). Hundreds of millions such decays occur per second in a typical clinical scan. Because the two photons arising from each annihilation travel in opposite directions, the rate of detection of such coincident pairs is proportional to the amount of emission activity, and hence glucose, along the line connecting the two detectors. In a PET camera the detectors are typically arranged in rings around the patient. By considering coincidences between all appropriate pairs of these detectors, a set of projection views can be formed each element of which represents a line integral, or sum, of the emission activity in the patient's body along a well defined path. These projections are typically organized into a data structure called a sinogram, which contains a set of plane parallel projections at uniform angular intervals around the patient. A three dimensional image of the radiopharmaceutical's distribution in the body can then be reconstructed from these data.
Most PET scans are performed using pure positron emitters, and can be made quantitative by performing normalization, attenuation correction and scatter correction processes on the acquired image data. Single gamma background can be removed from the image data acquisition of such pure positron emitter isotopes through the use of time-coincidence detection. However, there are isotopes that decay through the emission of a positron while the nucleus remains in an excited angular momentum state, leading to a prompt gamma emission (e.g., within about 0.1 nsec of the annihilation gamma pair in liquids or solids and 10-100 nsec in atmospheric air). This solitary gamma plus the two annihilation gamma photons (E=511 keV) derived from a positron-electron annihilation yields a triple of coincident gammas with known energies. Thus the net decay signature is a 511 keV gamma pair traveling in opposite directions and a solitary gamma with a non-correlated emission direction and distinct energy. When a non-standard PET isotope is used, therefore, the prompt gamma background component additionally must be compensated for in the acquired data.
Prior efforts have attempted to compensate for the prompt gamma component by using a flat background or a modeled prompt gamma distribution in the non-scatter tails of the sinogram representation of the acquired projection data. Such methods however have proven to be inaccurate.